The proposed project is a multicenter, multi-method longitudinal study of social network panels of active intravenous drug users (IVDUs). Long-term study objectives are: 1) to delineate factors influencing HIV transmission among IVDUs in community settings, 2) to evaluate longitudinally the impact of a model AIDS intervention program, and 3) to identify and explain the structural characteristics and social dynamics of IVDU networks which have direct bearing on the design and conduct of AIDS prevention programs targeted to this population. This study utilizes established relations with existing research cohorts to build upon preliminary findings from National AIDS Demonstration Research projects in Baltimore, Chicago, Denver, and El Paso. The multi-method research design incorporates a longitudinal panel survey and an ethnographic component. The panel survey gathers quantitative data to monitor AIDS risk behaviors, risk reduction adaptations, and HIV seroprevalence among 2,150 IVDUs in 17 network panels over a four year period. Ethnographic research methods focus on a systematic analysis of IVDU social networks as well as high risk behaviors of network members. Social network inquiry will generate typologies of interactional patterns within networks and specify the detailed factors influencing these interactions over time. Ethnographic analysis of high risk drug use and sex practices will illustrate the subcultural patterns, meanings, and contexts associated with high risk activities. In addition to addressing its primary aims, this study is expected to generate numerous collaborative investigations utilizing established research panels. Further, the intervention program is expected to serve as a model for public health agencies developing prevention service capabilities.